Conventionally, a synthetic resin bottle obtained by a process in which a preform with a bottomed cylindrical shape (an injection molding) is formed by injection molding by using a thermoplastic resin such as polyethylene terephthalate, and this preform is molded by biaxial stretch blow molding or the like has been generally used in a wide range of fields as a container that accommodates contents such as various beverages.
In such a synthetic resin bottle, in order to allow contents that are easily changed in quality by light to be accommodated, known is a technology of imparting light-shielding properties by compounding a colorant such as a pigment with the thermoplastic resin.
However, in recent years, used bottles are collected and recycled as a recycled resin in various applications. Under such circumstances, there is a problem that application of recycled resins is restricted in the case where a colorant is mixed into the resins. Therefore, as a technology of imparting light-shielding properties by distributing foams on the bottle wall without compounding a colorant, proposed is a technology in which a foaming gas such as an inert gas is dissolved in a thermoplastic resin to allow fine cells to be generated when a preform is produced by injection molding, and then, this preform is foamed by a heating and blowing process in biaxial stretch blow molding, whereby the preform is molded into a prescribed bottle shape.
On the other hand, one of the applicants of the present application previously proposed a technology of further improving light-shielding properties and suppressing deterioration in appearance that is caused by presence of foams (see Patent Document 1).
In such a technology, a foaming gas such as an inert gas is dissolved in a resin and generation of foams is suppressed at the time of injection molding of a preform to obtain a preform in a non-foamed state, and this preform is foamed by a heating and blowing process in biaxial stretch blow molding, whereby the preform is molded into a bottle shape that has a large amount of small cells being distributed therein.
On the other hand, as a method for injection molding the above-mentioned foamable injection molded product, a technology utilizing a 2-stage pre-plasticizing injection apparatus that comprises a plasticizing unit and an injection unit has been proposed (see Patent Document 2). In this technology, a molten resin in which a foaming gas such as an inert gas is dissolved in the plasticizing unit is injected into an injection mold by means of a reciprocally-moving plunger that is provided in an injection unit. This apparatus is provided with an accumulator in which the molten resin is accumulated through a conduit prior to the injection.